It is commonly required that a person use only one hand to operate a hypodermic syringe. As will readily be apparent, to operate a syringe on its forward stroke with one hand is much easier than to operate it on its rearward stroke, and it is particularly difficult to do so if the hand suffers from a lack of suppleness and dexterity due to age or arthritis. When, as so often happens, a person is both elderly and diabetic, and cannot rely on assistance to withdraw a sample of blood, it is especially important that the person be able to operate the syringe with confidence and safety.
Over the years, the problem of providing a hypodermic syringe assembly which may be used with only one hand has been attacked with numerous solutions. One of the earliest was a thumb ring attached to the end of the plunger and two finger rings attached to the syringe's barrel. In operation, the index and middle fingers were inserted in the finger rings on the barrel, and the thumb in the thumb ring. Such three-ringed syringes proved unsatisfactory, not only because they were awkward to use but because the syringe could not be properly stabilized while it was being used.
The awkwardness of the three-ring syringe was relieved by the assembly disclosed in U.S. Pat. No. 2,842,128 in which the finger rings on the barrel were dispensed with, in favor of an adjustable thumb ring which gripped the head of the plunger. Still later, in U.S. Pat. No. 3,316,909, the thumb ring was retained as an integral part of the plunger, and finger flanges were provided on the barrel to lend stability to the syringe. In both the foregoing prior art assemblies the thumb ring required the use of the thumb to withdraw the plunger and each was limited by the relatively short-radius curve of the movement of the thumb due to its physical articulation in the human hand.